In a historic move, Defense Secretary Ashton Carter said Thursday that he would finally end the Pentagon’s formal ban on women in combat jobs, allowing them to serve in all artillery, infantry and other frontline units for the first time in April.

Carter’s sweeping decision was a defeat for the Marine Corps, the last holdout in the battle to integrate women. It had sought exemptions for some jobs, such as machine gunner, arguing that women lacked the strength and ability to fight and survive in combat.

But Carter said at a Pentagon news conference that he had rejected the Marines’ requests because the military is a joint force and should operate under a common set of standards.

“There will be no exceptions,” Carter said. “This means that, as long as they qualify and meet the standards, women will now be able to contribute to our mission in ways they could not before.”

He gave the armed services until Jan. 1 to submit plans to open combat jobs and until April 1 to begin implementing the changes.

The decision marks a fundamental shift for the tradition-bound, male-dominated U.S. military, which has spent four decades slowly easing rules that restricted women in uniform.

But Carter said that after an intense review, the chiefs of the Army, Navy, Air Force and Special Operations Command had agreed to open all their combat positions, schools and units to qualified women.

Women long have sought an end to the Pentagon’s combat restrictions, which have prevented them from winning promotions as quickly as men. Combat experience is crucial for career advancement in the military.

More than 250,000 women have served as drivers, pilots, analysts and in hundreds of other military jobs in Iraq and Afghanistan since 2001, according to the Pentagon.

A total of 161 women have lost their lives and 1,015 have been wounded in action. Two women received Silver Star medals, one in 2005 and one in 2008, for action in battle.

The push to open more military jobs to women began amid the turmoil of the Vietnam War. The first major barrier fell in 1975 when West Point and other U.S. service academies were opened to women. They were permitted to serve on warships at sea in 1993.

In 2013, then-Defense Secretary Leon E. Panetta overturned a 1994 Pentagon rule that had restricted women from combat units. He gave the uniformed services until this fall to research the issue and then agree to open their ranks, or justify specific exemptions.

Since then, the military has opened 111,000 jobs that had been off-limits to women. That left about 220,000 others, including those in infantry, armor, artillery and some special operations units.

Those jobs will now open. For the first time, women who pass the rigorous training and testing will be able to join the Army Special Forces and other special operations units.

Carter’s decision was widely expected after two women graduated in August from the Army’s grueling Ranger School at Ft. Benning, Ga., but were not allowed to join the 75th Ranger Regiment, an elite light infantry and special operations unit.

“To succeed in our mission of national defense, we cannot afford to cut ourselves off from half the country’s talents and skills,” Carter said Thursday. “We have to take full advantage of every individual who can meet our standards.”

In a statement, President Obama compared the full integration of women in the armed services to the racial desegregation of the military decades ago, and more recent orders that allowed gays and lesbians to serve openly.

“I know that this change, like others before it, will make our military even stronger,” he said. “Our armed forces will draw on an even wider pool of talent. Women who can meet the high standards required will have new opportunities to serve.”

Critics and supporters in Congress and elsewhere were quick to respond.

Sen. John McCain (R-Ariz.) and Rep. Mac Thornberry (R-Texas), chairmen of the Senate and House armed services committees, said in a statement that they “intend to carefully and thoroughly review all relevant documentation related to today’s decision.”

Rep. Tammy Duckworth (D-Ill.), an Army helicopter pilot who suffered severe injuries in Iraq in 2004, applauded the end of what she called “antiquated military policies.”

“I didn’t lose my legs in a bar fight,” she said. “Of course women can serve in combat. This decision is long overdue.”

Nancy Duff Campbell, co-president of the National Women’s Law Center, one of many groups that had pushed for the change, called it “a thrilling day” for women.

“Thousands of women will now have the opportunity to be all that they can be and our nation’s military will be the stronger for it,” she said.

It’s unclear whether Congress will seek to amend the Selective Service Act, a law that requires all American males to register for conscription within 30 days of their 18th birthdays. Women are not required to register. Carter acknowledged that issue was unresolved, but said it would be in the coming weeks.

The decision to drop the ban came after a rare spat between the civilian and military sides of the Pentagon.

In September, the Marines released an executive summary of a 1,000-page study of women in combat skills tests. The so-called Integrated Task Force report had a range of findings but concluded that women hurt combat capability.

The Marines cited the study when they asked for exemptions. But Navy Secretary Ray Mabus challenged how the study was conducted, and Carter clearly agreed.

Gen. Joseph Dunford, who was Marine commandant when the study was conducted, but is now chairman of the Joint Chiefs, was noticeably absent during Carter’s announcement.

“He will be at my side as we do the implementation,” Carter said when asked about his nonappearance.

The decision to open all military jobs to women is the latest by the Obama administration to lift barriers against women and gay, lesbian, bisexual and transgender people seeking to serve in the armed forces.

In September, Obama nominated Eric K. Fanning as secretary of the Army. He is the first openly gay civilian to head a U.S. military service.

In June, the Pentagon gave gay and lesbian service members full protection from discrimination under an equal opportunity policy, meaning it will consider sexual orientation the same way it considers race, religion, color, sex, age and national origin when investigating complaints of discrimination.

“We will strive to ensure that the force of the future remains so long into the future,” Carter said. “Today, we take another step toward that.”